ARM Research Resolves Climate Model Controversy

Summary

Atmospheric Radiation Measurement (ARM) results published in the Journal of Geophysical Research (9 May 2003) and Science (20 June 2003) resolve a long-standing discrepancy between modeled and observed solar absorption. The controversy began with findings published in Science in 1995 that indicated that clouds absorb 40% more incoming solar energy sunlight than model calculations suggested they should. This phenomenon was called excess or anomalous absorption, which is defined as the difference between measured and calculated absorption. If anomalous absorption were true, the implications were that radiative transfer models, the component of models that calculates how gases and cloud droplets absorb, scatter, and reradiate solar radiation, were flawed. In the fall of 1995, the ARM program began its investigation of this scientific question with the Southern Great Plains field experiment, the ARM Enhanced Shortwave Experiment (ARESE). Unfortunately, there was only one day of acceptable conditions, and the results were inconclusive. Given the lack of resolution, ARM decided to host a second experiment, ARESE II, at its SGP site. In the interim following ARESE, ARM made improvements in the radiative transfer models to treat solar absorption and scattering in a detailed, explicit manner. The effects of aerosols were also included. Comparisons between results from ARESE II and the new generation models indicate that calculated absorption is accurate to about 5% and depends most critically on the specified aerosol type. This discrepancy is now within the error of the measurements.